Vehicle systems may include multiple coolant loops for circulating coolant through distinct sets of engine components. The coolant flow may absorb heat from some components (thereby expediting cooling of those components) and transfer the heat to other components (thereby expediting heating of those components). For example, a high temperature coolant loop may circulate coolant through an engine to absorb waste engine heat. The coolant may also receive heat rejected from one or more of an EGR cooler, an exhaust manifold cooler, a turbocharger cooler, and a transmission oil cooler. Heat from the heated coolant may be transferred to a heater core (for heating a vehicle cabin), and/or dissipated to the atmosphere upon passage through a radiator including a fan. As another example, a low temperature coolant loop may circulate coolant through a charge air cooler. When required (such as when cabin air conditioning is requested), coolant in the low temperature loop may be additionally pumped through the condenser of an air conditioning (AC) system to absorb heat rejected at the condenser by a refrigerant of the AC system. Heat from the heated coolant may be dissipated to the atmosphere upon passage through another radiator including a fan. One example of such a vehicle coolant system is shown by Ulrey et al. in US20150047374. Another example coolant system is shown by Isermeyer et al. in US20150040874. Therein a heat exchanger enables heat exchange between a charge air cooling coolant circuit and a refrigerant circuit of the condenser.
However, the inventors herein have identified potential issues with such coolant systems. As one example, if there is a loss of refrigerant from the AC system, adequate cabin cooling may not be provided. However, due to the AC system having the refrigerant mass split into liquid and gas states at any given time, and different regions of the AC system being at different temperatures at any given time, it may be difficult to diagnose the refrigerant level. In some vehicle systems, low refrigerant levels can be diagnosed in response to condensation patterns on the AC system evaporator. Additionally, a sight glass on the condenser outlet may be used to distinguish a drop in AC performance due to low refrigerant levels from other component degradation conditions, such as a worn AC compressor or a degraded condenser heat exchanger. As such, the drop in AC performance may alternatively be due to a temporary obstruction in refrigerant flow, such as due to a pinched line. However, for such diagnoses to be effective, the AC system may need to stabilize for a duration. As such, this may be difficult due to the limited access to the AC evaporator which tends to be deep within a vehicle's HVAC unit, under the instrument panel. Errors in reading the evaporator frosting pattern can result in incorrect diagnoses. In addition, the need for a sight glass adds significant cost. Consequently, in response to degraded AC performance, a service technician may erroneously replace AC components (such as the compressor, the condenser, a thermal expansion valve. an accumulator, etc.) instead of checking the fittings for a leak and adding refrigerant to the AC system.
In one example, the above issues may be better addressed by a method for a vehicle air conditioning system comprising: estimating a target coolant flow rate through a coolant circuit based on a cooling demand at each of an air-conditioning condenser, a charge air cooler (CAC) and a transmission oil cooler (TOC) of the coolant circuit; modeling a reference air-conditioning (AC) head pressure in a refrigerant circuit coupled to the condenser based on each of the target coolant flow rate and a coolant temperature; indicating degradation of the refrigerant circuit responsive to the reference AC head pressure relative to an actual AC head pressure; and in response to the indication, adjusting a ratio of coolant flow through the condenser relative to the CAC. In this way, refrigerant loss may be better identified and addressed.
As an example, each of a condenser of an AC system and a CAC may be coupled to distinct branches of a coolant circuit downstream of a proportioning valve, coolant directed into the circuit via a coolant pump. The condenser may be further coupled to a refrigerant circuit of the AC system. The AC condenser may be positioned towards a rear end of the under-hood area. During vehicle operation, as driver demand and cabin cooling demand changes, the apportioning of coolant flow to each branch may be varied. For example, when cabin cooling is demanded, a desired coolant temperature is determined. Then, by referring a 2D map or model that maps a relationship between the coolant flow rate, the desired coolant temperature, and an AC head pressure, taking into account parasitic losses, a target coolant flow rate through the AC condenser may be determined (as the point of minima of the asymptote of the 2D map). In particular, there may be a coolant flow rate above which the change in coolant temperature is not significant due to an increase in parasitic losses at the CAC. This coolant flow rate may be set as the desired coolant flow rate through the AC loop. In addition, a corresponding reference AC head pressure may be determined. Based on an error between the actual AC head pressure and the expected reference AC head pressure, refrigerant circuit issues may be identified, distinguished, and accordingly addressed. For example, when the actual head pressure remains less than the reference head pressure over a longer duration (resulting in a larger integrated error), it may be determined that the refrigerant line is pinched or clogged. Accordingly, coolant flow through the condenser may be reduced. As another example, when the actual head pressure remains less than the reference head pressure over a shorter duration (resulting in a smaller integrated error), it may be determined that the refrigerant line is leaking. Accordingly, coolant flow through the condenser may be increased. If the actual head pressure exceeds the reference head pressure, it may be determined that the AC condenser is working harder than expected. Accordingly, coolant flow through the condenser may be increased to improve AC cooling.
In this way, existing components may be used to monitor an AC system and more reliably diagnose AC system issues. By modeling the expected head pressure of the AC system and the comparing it to the actual pressure, the diagnostics can be performed in a shorter amount of time while relying on fewer components without compromising the accuracy of the results. In addition, erroneous diagnoses are reduced. By better distinguishing AC system errors, appropriate diagnostic codes may be set and mitigating actions may be performed in a timely manner. Overall, engine cooling performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.